Your school teacher is wrong here. He doesn’t have any right to discriminate against anyone on the basis of skin tone. India has come so far from those backward thought processes and has made so many laws to make everyone equal. Everyone has the right to equality as their fundamental right.
Indian Penal Code, 1860 (Section 153 A) – Criminalises the use of language that promotes discrimination or violence against people on the basis of race, caste, sex, place of birth, religion, gender identity, sexual orientation, or any other category.

To deal with this matter, you need to tell about this to an adult- Parents, Older siblings, guardians, or someone you trust. This is not a matter that you can handle alone, therefore, you need their support.

After talking with an adult, you should go along with an adult and talk to your school teacher. Try to have a good communication with your teacher and tell him/her to stop this behavior.

Then, wait for any change in his behavior. If his behavior doesn’t change, then, write a written complaint against your school teacher and take this matter to your school’s principal.